1. Field of the Invention
The invention relates to a downshift control apparatus of a vehicular automatic transmission and a control method thereof for suitably preventing occurrence of a shock caused by a downshift performed during an accelerating operation of a vehicle.
2. Description of the Related Art
For example, Japanese Patent Application Laid-open No. 9-229176 proposes a shift control apparatus that operates as described below at the time of a downshift, for example, a clutch-to-clutch downshift in which the engaging pressure of a disengage-side hydraulic friction engagement device is reduced and the engaging pressure of an engage-side hydraulic friction engagement device is temporarily raised. The shift control apparatus determines whether the present downshift is a downshift that requires a temporary increase in the engine rotation speed in order to realize an equal-speed downshift. The apparatus changes the content of the control of increasing the engaging pressure of the engage-side hydraulic friction engagement device between the case of a downshift that requires an increase in the engine rotation speed and the case of a downshift that does not require an increase in the engine rotation speed. Therefore, the engaging pressure of the hydraulic friction engagement devices that affects the tendency of change in the input shaft rotation speed of the automatic transmission is controlled in accordance with the control of the engine rotation speed. Hence, the torsional oscillation of the power train during an end-of-shift period decreases, and the shock caused by a shift is reduced.
In some vehicles, a moderate throttle opening degree control for maintaining a degree of throttle opening that is less than the degree of throttle opening corresponding to an accelerator pedal operation for a predetermined period following the beginning of the accelerator pedal operation, is performed in order to mitigate the acceleration shock to the vehicle. In such a vehicle, during a predetermined period in an accelerating operation, the degree of throttle opening is restricted so that the torque input to the automatic transmission is reduced. Therefore, the engaging pressure of an engage-side hydraulic friction engagement device or the engaging pressure of a disengage-side hydraulic friction engagement device controlled (set) corresponding to the torque input to the automatic transmission is reduced. Hence, when the degree of throttle opening is set at a value corresponding to the amount of accelerator operation simultaneously with the end of the moderate throttle opening degree control, there is a danger of insufficient engagement torque of the hydraulic friction engagement devices, which may impede smooth takeover of engagement torque during the clutch-to-clutch downshift, and therefore may cause a shock.